This invention relates to a knock detecting apparatus adapted for use with internal combustion engine ignition timing control systems and having the function of detecting knocking of an engine from the vibrations caused inside and outside the engine cylinders due to the cylinder pressure and adjusting the ignition timing.
The existence of a close correlation between the ignition timing and the cylinder pressure is well known in the art. When the mixture is exploded, if there is no knocking, there will be no superposition on the cylinder pressure of higher harnonics (those including components of a frequency band which is usually determined by the diameter of the engine cylinder bore and the velocity of sound in combustion and the harmonics are produced by the intermittent and rapid burning of the mixture). If the engine starts knocking, such harmonic components start to be superposed on the cylinder pressure when the crank angle reaches around the point of the maximum cylinder pressure value and this results in the production of vibrations or sound on the outside of the cylinders. A study of the cylinder pressure signals generated inside the cylinders as well as the manner of production of vibrations or sound outside the cylinders has shown that the beginning of knocking (or trace knock) starts occurring at the engine crank angle at which the cylinder pressure attains the maximum value, and when the knock gradually increases in intensity (to light knock and heavy knock) such harmonic components start to be superposed greatly on the cylinder pressure on the advanced side (or on the ignition side) with respect to the maximum value cylinder crank angle.
In this case, it has been considered that the frequencies of the higher harmonics due to the knocking are determined linearly with the cylinder bore diameter and the velocity of sound in combustion and it has been said that the frequencies occur only in a specified frequency band (usually in the range of 7 to 10 kHz). Knock detecting apparatus known in the art are so designed that the ignition timing is controlled by detecting the occurrence of knocking on the understanding that the knock can be detected by simply detecting the vibrations produced outside the engine or noting only the specified frequencies. However, it has been determined that the detection accuracy of this type of apparatus tends to deteriorate depending on the operating conditions of the engine and there is a certain region where the detection of weak or trace knock is difficult. More specifically, if the knock is detected from the frequencies in the 7 to 10 kHz band, at the high speed operation of the engine the resulting vibration noise (e.g., the valve seating vibration) in the engine body increases and the S/N ratio is deterioated. The effect of these phenomena is so large that particularly during the periods of high speed and load operation, the detection of trace knock will be made practically impossible. If the control of the ignition timing is enforced despite the deteriorated S/N ratio, an excessively intense knock will be caused and sometimes damages to the engine body including burning loss of the spark plugs will be caused. In the past, generally it has been necessarily to stop the ignition timing control during the periods of high speed and high load operation so as to prevent such damages. In the case of known methods in which the detection of vibrations is simply attempted without noticing any specific frequency band, the effect of vibration noise has been so large that the detection of trace knock has been practically impossible and thus it has been impossible to expect smooth control of the ignition timing.
Examination of various knock detecting methods designed to overcome the deficiencies in the known methods has shown that knocking frequencies are not confined to any single frequency band but knocking occurs in a plurality of frequency bands, and that if these frequency bands are taken into consideration, it will be possible to reduce the danger of superposition of engine vibration noise and to effect the detection of knock even during the periods of high speed operation with greater accuracy.